This invention relates to hot runner systems for injecting plastics materials from a plasticizer unit into an injection mold and, in particular, to hot runner systems suitable for molding very small plastic parts and devices.
Although a number of attempts have been made by various companies to develop systems for efficiently and economically producing small plastic parts such as those that might be used in medical syringes, laboratory ware and surgical equipment, there remains a need to develop improved systems to make micro plastic parts which are not unduly expensive and which are adaptable for the molding of a variety of parts and components. One known system for making small parts is that made by Mold Hotrunner Solutions Inc. of Georgetown, Ontario, Canada, and this uses a multi-nozzle for injecting the plastic material into the mold using several gates arranged on a circle. This arrangement is able to produce a plastic part that consists of a concentric ring. With this system larger parts can be made using individual nozzles because the flow residence time is less critical.
A recently developed German system involves encapsulating an electronic subassembly for a sensor with a protective housing using injection molding. In this recently developed system a prefabricated subassembly is passed through an injection molding machine while still on a metal strip and this is encapsulated using a so-called “outsert technique”. Unlike the conventional hotrunner system, this new system has a local plunger injection system positioned at the back end of each needle valve. This consists of a tubular plunger which acts on a hollow cylinder or antechamber between a 2 millimeter thick needle and a 3.5 millimeter diameter runner. As a result the feeding of the plastic melt with the aid of the plasticizing unit is entirely separate from the injection into each of the cavities. This process is also known as transfer injection molding. This known system is taught in recent German patent No. 196 32 315.
In the first step of this injection molding process or cycle, the needles of the needle valve nozzles keep the gate closed. The injection stroke of the screw forces the plastic melt out of the screw antechamber via the gate bushing, the two valves and the runners into the antechambers around the valve needles. As a result the right amount of melt for the injection is ready in each plunger injection unit. In the next step, a mounting plate moves in the direction of the runner whereby the valves close the connection to the plasticizing unit. Thus the plastic melt is enclosed in the runner system and can no longer flow back to the machine nozzle. Through the movement of a plate and a spring assembly behind the tubular plunger, a melt pressure of 600 to 800 bar is built up. Then another plate assembly travels with the needle valves to open the valve gate to the component. At the same time the first mentioned movable plate continues its movement so that the tubular plunger can inject the melt located in the hollow cylinder into the cavity, the injection pressure being around 2500 bar. After the required holding pressure time and cooling time have elapsed, both of the plate assemblies return to the original position and the needle valves close off the gate. The valves clear the way for the melt that has been plasticized so that a new cycle can begin.
U.S. Pat. No. 6,403,010 issued Jun. 11, 2002, also describes an apparatus for molding micro parts out of plastic. This apparatus includes a plasticizing apparatus, a dosing or metering apparatus connected by a passage to the plasticizing apparatus, and an injection apparatus connected to the dosing apparatus by a further passageway. Backflow of the melt from the metering apparatus is prevented by a non-return valve.
U.S. Pat. No. 4,863,369 issued Sep. 5, 1989 to Husky Injection Molding Systems Ltd. describes a hot runner system for delivering melted plastics material from a plasticizer unit to a multi-cavity mold. This system includes a metering device in the form of a piston mounted in a cylindrical chamber which is individual to each mold cavity and which is designed to feed a desired quantity of the thermal plastic material to each cavity. However, this hot runner system, because of its particular arrangement and conduit system, is not believed to be suitable for molding very small plastic parts and devices. Also, in this known system, after the reservoir in each metering device has been fully charged with the plastics compound, a valve is closed upstream of the metering device. Only after the valve is closed is a nozzle stem of an injector opened and the piston advanced in order to force the plastics compound through a conduit and an injector passage into the mold cavity.
There are a number of known problems that must be taken into account in the design of hot runner systems. One serious potential problem is thermal degradation of the melted plastics material caused by excessive residence time in the delivery channel or conduit. Such degradation can cause changes in the molded parts physical properties or lead to burning. Also, mold designs for small parts and devices include tight cavity spacing and large pockets cut into the mold base relative to its size. These molds require cooling and cavity spacing for small parts can limit water line access to the cavities in the gate area. Consideration for the proximity and size of water lines is critical for molding the parts. Furthermore, small part molding relies on heat input from manifold heaters for melt uniformity and thus heater layout and thermal conductivity must be considered in manifold design to obtain minimal temperature variation across the manifold.
A further challenge for the micromolding industry is that the molding presses and the injecting system for micro parts generate higher injection speeds and pressures to push the melt through tiny nozzles and flow channels. For example, normal plastic molding can use an injection pressure of about 20,000 psi while some known micromolding applications need up to 40,000 psi. Higher injection speed helps reduce viscosity through shear thinning and ensures that the material will fill the part before it cools.
Adding to the challenges of micromolding, high performance miniature parts often require engineered materials like polyamide-imide, liquid crystal polymer, PEEK, and PPS, in addition to more standard ABS, nylon, and acetal.
Generally speaking, a micromolding system demands a small extrusion screw for the plasticizing unit, this screw being proportioned to the shot size. Generally, a 14 mm screw is the smallest root diameter used for micromolding purposes. It should also be noted here that runner systems for micromolding in the past have tended to be cold-runner types but the substantial disadvantage of such systems is that there can be a significant amount of material wastage.
According it is an object of one aspect of the present invention to provide an improved hot runner system for injecting plastics material from a plasticizer unit into an injection mold, this system being reliable and relatively easy to maintain and use as well as being adaptable for the molding of very small plastic parts and devices.
An object of another aspect of this invention is to provide a hot runner system for injecting melted plastics material into at least one small mold cavity which employs a relatively simple, reliable check valve, including a valve chamber and a ball movable within this chamber between a valve closing position and a valve open position, this valve preventing the melted plastics material from backflowing to the plasticizing unit during operation of a metering apparatus located downstream of the check valve.
An object of a further aspect of the invention is to provide a unique hot runner system for injecting melted plastics material into at least one mold cavity, this system employing a two-part manifold apparatus wherein first and second manifold sections are spaced apart by an insulating arrangement, each section having its own heater so that the first section can operate at a lower elevated temperature range than the second manifold section.